#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
int main(){
    int n;
    cin>>n;
    vector<int> a(n);
    for(int i=0;i<n;i++){
        int x,y;
        cin>>x>>y;
        a[i]=y-x;
    }
    sort(a.begin(),a.end());
    if(a[0]==a[n-1]){
        cout<<a[0]<<endl;
    }else{
        cout<<-1<<endl;
    }
}